1. Field
The present invention is directed to ceramic compositions and, more specifically, to a silicon carbide ceramic composition and method of making it through liquid phase sintering.
2. Related Art
Silicon carbide ceramic materials are used in a variety of applications requiring good heat resistance, strength and tribological properties. For example, silicon carbide is often used in automotive and industrial applications, such as in fluid seals. However, the limited toughness of silicon carbide has limited its utility in applications requiring this property.
Traditionally, silicon carbide is sintered by solid state sintering. Typically, solid state sintering employs boron and carbon as sintering aids. Solid state sintering is traditionally performed at about 2150xc2x0 C. and yields a relatively high sintered density of about 96-98% of the theoretical density (of solid silicon carbide). The crystalline structure produced by solid state sintering, however, is undesirable for some applications. Specifically, solid state sintering results in an equiaxed crystalline microstructure with relatively low fracture toughness (approximately 2.5 MPa m1/2 as measured by the indentation crack length method, ASTM Test No. C1421).
It has also been discovered that silicon carbide can be processed by liquid phase sintering in a manner similar to silicon nitride. Liquid phase sintering is traditionally performed at about 1750-2000xc2x0 C. In liquid phase sintering, a rare earth metal oxide and alumina are typically used as sintering aids. The rare earth metal oxide and alumina form a liquid glass as the temperature is elevated during sintering. The liquid phase pulls silicon carbide particles together through capillary action. Smaller silicon carbide particles are dissolved into the glass phase and precipitate onto larger particles, densifying the material. Liquid phase sintering results in silicon carbide having elongated (acicular) crystalline microstructure. The acicular microstructure improves the fracture toughness of silicon carbide produced by liquid phase sintering over silicon carbide produced by solid state sintering by as much as two to three times (up to approximately 6 MPa m1/2 as measured by the indentation crack length method).
Liquid phase sintering is generally performed with xcex2-phase silicon carbide powder, which is typically more expensive than the xcex1-phase powder that may be used in the solid state sintering process. During the liquid phase sintering process, the xcex2-phase transforms into elongated xcex1-phase grains, improving toughness. The xcex1-phase silicon carbide powder may be used in the liquid phase sintering process, but density and toughness are compromised. Where xcex1-phase silicon carbide is used in a liquid phase sintering process, higher sintering temperatures (up to about 2050xc2x0 C.) and hot pressing may overcome the density and toughness problems. However, hot pressing leads to increased cost and sintering temperature that leads to a thicker reaction layer, which often must be machined off, further increasing the cost.
In one embodiment, the present invention is directed to an unsintered ceramic body including at least one liquid phase sintering aid. The unsintered ceramic body further includes a boron containing compound, a free carbon containing compound, and silicon carbide.
In another embodiment, the present invention is directed to a method of making a sintered ceramic body. The method includes combining at least one liquid phase sintering aid, a boron containing compound, a free carbon containing compound, and silicon carbide to form a green ceramic, shaping the green ceramic into a ceramic body, and sintering the ceramic body.